System for controlling flow through a process region

ABSTRACT

A regulator for maintaining a constant partial vacuum in a region, such as in a piece of process equipment or a fume hood. The regulator includes a path, through which fluid passes from the region to the vacuum source, and a movably mounted piston having a frontal face, which is exposed to fluid in the path, and a distal face, which is exposed to a reference pressure, such as the pressure of the environment in which the equipment is located. The piston is disposed in the path so that the piston may constrict the path at a constriction point. Fluid in the path upstream of the constriction point exerts a pressure on the frontal face of the piston that tends widen the path at the constriction point. The piston is mounted so that the weight of the piston exerts a force on the piston in a direction that tends to widen the path at the constriction point. A force is exerted on the piston, preferably by a spring under compression, in a direction that tends to narrow the path at the constriction point. A buffering gas, introduced to the area between the piston&#39;s skirt and the piston mounting structure, may be used to keep noxious fumes from backing up into the environment and/or to provide an efficient bearing.

DESCRIPTION

This application is a continuation-in-part of application Ser. No.07/965,909, now abandoned, and application Ser. No. 07/965,907, now U.S.Pat. No. 5,320,124, both of which were filed Oct. 23, 1992, and both ofwhich are continuations-in-part of application Ser. No. 07/850,767 (nowissued as U.S. Pat. No. 5,251,654), application Ser. No. 07/851,017(which will shortly issue as U.S. Pat. No. 5,255,710), application Ser.No. 07/852,084 (which will shortly issue as U.S. Pat. No. 5,255,709) andapplication Ser. No. 07/851,016 (now issued as U.S. Pat. No. 5,220,940),all filed Mar. 13, 1992, and all of which are continuations-in-part ofapplication Ser. No. 07/669,746, filed Mar. 15, 1991, now abandoned, andwhich is a continuation-in-part of application Ser. No. 07/405,835,filed Sep. 11, 1989 and issued Mar. 19, 1991 as U.S. Pat. No. 5,000,221,which is a continuation-in-part of application Ser. No. 07/178,505,filed Apr. 7, 1988, now abandoned. The present application is also acontinuation-in-part of application Ser. No. 08/126,151, filed Sep. 23,1993. All these applications are hereby incorporated herein byreference.

TECHNICAL FIELD

This invention generally relates to a device for regulating the flow ofa fluid, particularly a gas, through the device.

BACKGROUND ART

In heating, ventilating, and air conditioning (HVAC) systems and houseexhaust systems, air flow is typically controlled using resistors toslow down the flow of air to and from different points in a building.When one resistor is adjusted, the pressure level throughout the systemwill change; any change in the system pressure will affect the flow ofair past every other resistor. Thus, adjusting a resistor at one pointcauses "cross-talk" with resistors at other points.

One of the most complex problems confronted by the HVAC industry iscontrolling air flow through process rooms, such as the clean rooms usedin semiconductor integrated-circuit chip manufacturing, or the medicaland biotechnology laboratories kept at below atmospheric pressure toprevent potentially dangerous microbes from blowing out of thelaboratories. Some air exits the process room through process equipmentand other work stations with fume hoods. A partial vacuum is usuallyrequired in such equipment in order to ensure that noxious fumes ordangerous microbes do not leak from the process equipment or fume hoodsand thereby endanger personnel working nearby. It is frequentlyimportant that a constant partial vacuum be maintained in the processequipment in order to minimize defects in the integrated circuit chipsbeing manufactured. In some process equipment it is important that onlya small partial vacuum be maintained.

SUMMARY OF THE INVENTION

The present invention regulates a partial vacuum in a region, such as ina piece of process equipment or a fume hood. The partial vacuum may bewith respect to the environment in which the equipment is located, sothat the region's pressure is between the pressure of the environmentand the pressure of the vacuum source to which the regulator isattached. One embodiment of the invention is particularly well suitedfor maintaining at a fairly constant level a small partial vacuum in theregion with respect to the environment--i.e., a small pressuredifferential between the region and the environment.

The invention includes a path, through which fluid passes from theregion to the vacuum source, and a movably mounted piston having afrontal face, which is exposed to fluid in the path, and a distal face,which is exposed to a reference pressure, preferably the environment'spressure. Preferably, the areas of the frontal and distal faces areabout the same.

The piston is disposed in the path so that the piston may constrict thepath at a constriction point, which is preferably located between therim of the piston and the rigid wall of the fluid path. Fluid in thepath upstream of the constriction point (the region side of theconstriction point) exerts a pressure on the frontal face of the piston.The reference pressure is exerted on the distal face of the piston.Fluid in the path downstream of the constriction point (thevacuum-source side of the constriction point) exerts a pressure on thepiston in a direction transverse to the piston's direction of movement,so that pressure changes downstream of the constriction point have aminimal effect on the regulated partial vacuum. In one preferredembodiment, the piston is disposed in the path so that the fluid flowsradially outward from the frontal face's central portion towards thepiston's rim before flowing through the constriction point.

In an embodiment for creating a small partial vacuum, The piston ismounted so that the weight of the piston exerts a force on the piston ina direction that tends to widen the path at the constriction point, anda force is exerted on the piston, preferably by a spring undercompression, in a direction that tends to narrow the path at theconstriction point.

The spring's base may be made movable with respect to the portion of thepath at the constriction point in order to adjust the partial vacuum orto compensate for a varying flow rate. The spring's base may be moved soas to decrease or increase the apparent weight of the piston. As notedabove, to decrease the apparent weight of the piston, a force is exertedon the piston in a direction that tends to narrow the path at theconstriction point. In order to increase the apparent weight of thepiston, a force is exerted on the piston in a direction that tends towiden the path at the constriction point.

In one preferred embodiment, the invention includes a piston movablymounted adjacent the passageway. The piston may constrict the flow at aconstriction point between the rim of the piston and a wall of thepassageway. As the piston moves, the resistance the piston applies tothe flow through the passageway varies. The piston is mounted on or in apiston-mounting structure, such that the piston and the piston-mountingstructure define a reference-pressure chamber. A buffering fluid issupplied to a point between the piston and the piston-mountingstructure, such that one portion of the buffering fluid flows into thepassageway and another portion into the reference-pressure chamber. Thepiston may include a skirt which surrounds a portion of thepiston-mounting structure, and the point to which the buffering fluid issupplied is between the piston-mounting structure and the piston'sskirt. (In the preferred embodiment, the skirt is located outside of thepiston-mounting structure; however, in an alternative embodiment thepiston-mounting structure may be located outside of the skirt.) Thefirst portion of the buffering fluid flows into the passagewaydownstream of the constriction point.

The piston-mounting structure may be tilted from a vertical orientation,so that the force exerted by the weight of the piston in the directionthat the piston can move is less than the weight of the piston. Thisembodiment is well suited for maintaining at a fairly constant level asmall partial vacuum in the region with respect to theenvironment--i.e., a small pressure differential between the region andthe environment.

A biasing fluid may be introduced into the passageway upstream of theconstriction point, so as to ensure that there is always a substantialflow rate through the device. A dashpot may be used in order to reduceany harmonic vibrations of the piston.

In an alternative embodiment, the piston may be hingedly mounted so asto rotate around a hinge point, so that the weight of the piston createsa first torque urging the piston to turn around the hinge point, andmeans for creating a second torque in the opposite direction of thefirst torque is provided. The means for creating the second torque maybe a spring, a counterweight, a stalled DC electric motor or some othermeans.

The system for controlling the flow through the process region mayinclude a pressure transducer, which is protected from potentiallydamaging fumes by a buffering-gas flowing through a passagewayconnecting the transducer to the region. The system may also use acondensation trap using peltier elements, in order to remove some of thefumes before they enter the regulator and the house exhaust system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a device that may be used to maintain ina piece of process equipment a partial vacuum.

FIG. 2 shows a cross-section of a device that may be used to maintain ina piece of process equipment a small partial vacuum.

FIG. 3 shows how the device of FIG. 2 may be attached to the processequipment, the environment and the vacuum source.

FIG. 4 shows a cross-section of another device that may be used tomaintain a partial vacuum and that has a hingedly mounted piston.

FIG. 5 shows a cross-section of a device similar to the FIG. 2 device,but has a piston that is hingedly mounted.

FIG. 6 shows a cross-section of a device which by use of a buffering gasensures that fume-laden air coming from a fume hood or other equipmentcannot leak through the reference-pressure port.

FIG. 7 shows a cross-section of a regulator that uses a buffering gasalong with a spring which may be mounted on a base which is adjustable.

FIG. 8 shows a spring that may be used in the FIG. 7 regulator.

FIGS. 9A and 9B show side and top views of a spring-end connector, andFIG. 9C shows a top view of the connector with a spring attached.

FIG. 10 shows a dashpot for use between a piston and a piston-mountingstructure

FIG. 11 shows a system for controlling gas flow through a process region(e.g., the area inside a fume hood or a piece of process equipment whereprocess steps take place) to an exhaust.

FIG. 12A shows a condensation trap, which may be used in the systemshown in FIG. 11.

FIG. 12B shows a top view of a thermoelectric cooler, which may be usedin the condensation trap shown in FIG. 12A.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 shows a regulator as described in U.S. Pat. No. 5,000,221, issuedto the inventor of the present invention. The regulator is attached to apiece of process equipment, a fume hood or other region which is locatedin a process room or other environment. Air flows from the environmentinto the equipment, where noxious gases, microbes or other potentiallydangerous contaminants may be picked up by the air. Typically, there issome varying impedance on the flow from the environment into theequipment. For instance, the equipment may have a door, which is openedto permit the loading of some item which is to be treated by theequipment. When the door is opened, the impedance to the flow from theenvironment into the equipment is lessened. After passing through theequipment, the air is then pulled into the regulator's inlet 4, throughthe regulator, and then out of its outlet 8 to the vacuum source.

This regulator includes a piston 2 having a frontal face 15 and a distalface 16, which have approximately the same areas. The piston 2 ismounted on a mounting structure 65, so that it can move up to constrictthe flow through the regulator at a constriction point 30 between therim of the piston 2 and the housing 3 (which functions as a valve seat),and so that the piston 2 can move down to widen the path at theconstriction point 30. The piston's frontal face 15 is exposed to plenum13, through which the air flows just before passing through theconstriction point 30. The piston's distal face 16 is exposed to areference chamber 17, which is preferably attached by means of referenceport 9 to the environment, so that the fluid pressure on the piston'sdistal face 16, the reference pressure (P_(REF)), is equal to theenvironment's pressure.

Since the fluid pressure just downstream of the constriction point 30applies a force on the skirt of the piston 2 that is, for the most part,perpendicular to the directions in which the piston 2 can move, theregulator can maintain a fairly constant pressure differential betweenthe plenum 13 and the reference-pressure chamber 17, the pressure in theplenum 13 being less than the reference pressure. This pressuredifferential is related to the weight of the piston 2 and the surfaceareas of the frontal face 15 and distal face 16:

P_(REF) ·A=P₁₃ ·A+W, where A=Area of frontal face, which issubstantially equal to the area of distal face;

P₁₃ =Plenum pressure; and

W=Weight of piston 2.

Thus, ΔP=W/A, where ΔP=Pressure differential between plenum 13 andreference-pressure chamber 17.

The constant pressure differential can be used to maintain in theequipment to which the regulator is attached a constant partial vacuumwith respect to the environment. Such a constant partial vacuum can bemaintained by simply attaching the reference port 9 to the environment,and attaching the regulator's input 4 to the equipment so that there isonly a small pressure drop from the equipment to the plenum 13. Withsuch an arrangement, the flow rate through the regulator varies in orderto maintain the constant vacuum.

To maintain a smaller constant partial vacuum, a lighter and widerpiston 2 may be used. (Conversely, a heavier piston 2 may be used tocreate a larger constant partial vacuum.) However, there are practicallimits on how light the piston material can be. Another way ofdecreasing the partial vacuum in the equipment is by placing a valveupstream of the regulator (and downstream of the equipment), as shown inFIG. 1, to create a pressure drop from the equipment to the plenum 13.This valve may be adjusted by a stepper motor to alter the partialvacuum. This arrangement is however somewhat less effective formaintaining a constant partial vacuum and is better suited formaintaining a substantially constant flow rate.

The device shown in FIG. 2 maintains a small, substantially constantpartial vacuum very well. Like the device shown in FIG. 1, the FIG. 2device has a piston 2, movably mounted so that it may constrict the flowpath between the rim of the piston 2 and the housing 3 at a constrictionpoint 30. Air in the path downstream of the constriction point 30 exertsa pressure on the piston 2 in a direction perpendicular to the piston'sdirection of movement. The force of the reference pressure on the distalface 16, which is offset by the force exerted by air in the plenum 13 onthe piston's frontal face 15, tends to narrow the flow path at theconstriction point 30. The areas of the frontal face 15 and the distalface 16 are substantially the same, although it may be preferable insome embodiments to make the frontal face's area slightly smaller thanthe distal faces, as discussed below. (It is preferred that the frontaland distal faces be flat and perpendicular to the piston's direction ofmovement. If, however, the frontal and distal faces are not flat planesperpendicular to the direction of movement of the piston 2, theeffective areas of the faces--i.e., the projection of the faces on aplane perpendicular to the direction of movement of the piston 2--shouldbe equal.)

The piston 2 is mounted so that its weight exerts a force on the pistonin a direction that tends to widen the path at the constriction point30. Since weight of the piston 2 tends to widen the constriction point30, and tends to restore the regulator to its most open position whenthere is no flow, the weight of the piston 2 is considered a restoringforce.

Unlike the device shown in FIG. 1, the regulator shown in FIG. 2 doesnot have an adjustable valve mounted upstream of the plenum 13. Instead,the FIG. 2 regulator has a spring 21 mounted between the piston's distalface 16 and a base 22, which is adjustable. The spring 21 pushes upwardon the piston and makes the piston 2 act like a lighter piston.Normally, the spring 21 is under compression; however, the spring base22 may, in some embodiments, be pulled down far enough that the spring21 is put under tension, thereby making the piston 2 seem heavier. Thisarrangement is preferred to an alternative arrangement where the springis connected to the piston's frontal face 15 and pulls up on the piston2. In the preferred arrangement shown in FIG. 2, the spring 21, beingmounted inside the piston 2 and the piston-mounting structure 65, isprotected from the corrosive effects of the fume-laden air coming fromthe process equipment.

When the system is at equilibrium--i.e., when the partial vacuum is atthe desired strength and the flow rate through the regulator is notvarying--the force applied by the spring 21 acting against the weight ofthe piston 2 should be less than the weight of the piston 2 in order toreduce the apparent weight of the piston 2, and thereby reduce thepartial vacuum. In this system the forces balance out as follows:

    P.sub.REF ·A+F.sub.SPRING =P.sub.13 ·A+W

where

A=Area of frontal face exposed to fluid upstream of the constrictionpoint, which is substantially equal to the area of distal face;

P₁₃ =Plenum pressure;

F_(SPRING) =Force applied by spring 21; and

W=Weight of piston 2.

Thus, ΔP=(W-F_(SPRING))/A

where ΔP=Pressure differential between plenum 13 and reference-pressurechamber 17.

FIG. 3 shows how the regulator shown in FIG. 2 can be attached to apiece of equipment (or other region) in a process room (or otherenvironment). The regulator's outlet 8 is attached to the vacuum source.Its inlet 4 is attached to the equipment in such a way that there isonly a very small pressure drop from the equipment to the plenum 13 ofthe regulator. The reference port 9 is attached to the environment, sothat the reference pressure is the environment's pressure. Thus, air mayflow from the environment, through the equipment and the regulator, tothe vacuum source. Preferably, the path is not vented to the environmentat any point between the inlet 4 and the outlet 8, so that substantiallyall the fluid that flows through the inlet also flows through theoutlet.

The system quickly adapts to changes. If flow into the equipment fromthe environment is further impeded (such as when a door to the equipmentis closed), the pressure in the equipment drops momentarily. Theequipment pressure would remain at the lower level if the equipment wasattached directly to the vacuum source without the regulator. With theregulator in the flow path between the equipment and the vacuum source,a pressure drop in the equipment tends to cause a pressure drop in theplenum 13, but the piston 2 rises to maintain a constant pressuredifferential between the plenum 13 and the reference-pressure chamber17. The piston 2 in the higher position increases the flow impedancebetween the plenum 13 and the vacuum source, thereby counteracting veryquickly the pressure drop in the equipment.

Likewise, the decreasing of the impedance to the flow from theenvironment into the equipment (such as when a door on the equipment isopened) causes a momentary increase in the pressure in the equipment andcauses the piston 2 to move to a lower position. The piston 2 in thelower position decreases the impedance between the plenum 13 and thevacuum source and counteracts any transient pressure increase in theequipment.

Similarly, an increase in the environment's pressure causes the piston 2to rise to maintain a constant pressure differential between the plenum13 and the reference-pressure chamber 17. The piston 2 in the higherposition increases the impedance on the flow between the equipment andvacuum source, thereby increasing the pressure in the equipment andmaintaining a fairly constant partial vacuum in the equipment. Likewise,a drop in the environment's pressure causes the piston 2 to drop tomaintain the constant pressure differential between the plenum 13 andthe reference chamber 17 and thereby maintain the constant pressuredifferential between the equipment and the environment--i.e., theconstant partial vacuum in the equipment.

The regulator shown in FIGS. 2 and 3 can also maintain a fairly constantpartial vacuum in spite of fluctuations in the strength of the vacuumsource, which can happen when "cross-talk" between flow regulatorsoccurs--i.e., when the flow rate in other, parallel fluid paths to acommon vacuum pump fluctuates. An increase in the vacuum source'sstrength would cause a decrease in the equipment's pressure if theregulator was not attached between the vacuum source and the equipment.However, in the FIG. 3 system the onset of the increased vacuum sourcestrength causes a minute decrease in the pressure in the plenum 13,which in turn causes the piston 2 to rise, which increases the flowimpedance between the plenum 13 and the vacuum source, therebyoffsetting the increased strength of the vacuum source. Likewise, adecrease in the vacuum source strength causes the piston 2 to drop,which decreases the regulator's flow impedance, offsetting the decreasedstrength of the vacuum source.

Thus, the system shown in FIGS. 2 and 3 provides very rapid responses tofluctuations in the equipment's flow impedance, the environment pressureand the vacuum source's strength.

Certain conditions are required for the proper functioning of theregulator. In order to obtain the desired partial vacuum, the vacuumsource must be strong enough to create the necessary pressuredifferential between the regulator's plenum 13 and reference-pressurechamber 17. Also, the impedance between the environment and theequipment cannot be too high or too low. For example, if the equipmentis hermetically sealed from the environment, the piston 2 will be pulledup as far as it can go, creating as much flow impedance as it can,perhaps even sealing the plenum 13 off from the vacuum source. In such asituation, air from the reference-pressure chamber 17 could be suckedbetween the piston 2 and the piston-mounting structure 65, unless thereis an effective seal (such as a rolling diaphragm) between the piston 2and the piston-mounting structure 65. As is discussed in greater detailbelow, a biasing gas may be introduced into the fluid path between theequipment and the regulator, so that there is always some minimum flowthrough the regulator.

If there is too little impedance between the environment and theequipment, the piston 2 may be forced down as far as it can go, creatingas little impedance as it can. In such a situation, the partial vacuumin the equipment may not be as large as desired.

The force applied by the spring 21 on the piston 2 will typically varyaccording to Hooke's Law as the spring is extended or compressed fromits equilibrium position. In the regulator shown in FIG. 2, the spring21 is located inside the piston 2 and the piston-mounting structure istypically under compression. When the system is at equilibrium with flowpassing through the regulator, the forces acting on the piston 2--theplenum pressure times the frontal face's surface area exposed to fluidupstream of the constriction point, the reference pressure times thedistal face's surface area, the piston's weight and the upward forceapplied by the spring 21--balance out, so that the piston 2 floatswithout changing position. As the piston 2 rises and falls, in responseto changes in the vacuum source strength, the reference pressure or theequipment's flow impedance, the force applied by the spring accordinglydecreases or increases according to Hooke's Law (assuming that thespring's base 22 is not moved).

Since the spring force changes based on the position of the piston 2with respect to the spring base 22, the pressure differential betweenthe plenum 13 and the reference-pressure chamber also changesaccordingly:

ΔP=(W-k·x)/A, where k=spring constant, and x=amount spring iscompressed.

The varying spring force can be compensated somewhat by altering theeffective area of the piston's frontal face 15 so that the ratio of thefrontal face's area to the area of the distal face 16 is reducedsomewhat. It is preferred to alter the geometry of the constrictionpoint, so that a portion of the top of the piston lies effectivelybeyond the constriction point and is therefore not directly exposed tothe plenum pressure, but rather is exposed to the vacuum downstream ofthe constriction point.

As shown in FIG. 7, by altering the rim 23 of the piston 2 in this waythe ratio of the areas of the frontal and distal faces (exposedrespectively to the plenum 13 and the reference chamber 17) can be mademuch closer to one. In addition, this geometry balances the pressureexerted by the gas on the bottom of the skirt 29. It is preferred thatthe gas pressure being exerted upwardly on the bottom of the skirt 29 bebalanced by gas pressure downstream of the constriction point on aportion of the top of the piston 2 that is of substantially equal areaas the bottom of the piston's skirt 29. Since the plenum pressure isgreater than the pressure at the exhaust 8, an altered rim 23 ispreferred to an unaltered rim, such as that shown in FIGS. 1, 2 and 6,so as to minimize the effect of changes in the exhaust pressure on theplenum pressure. When a spring 21 is used to reduce the apparent weightof the piston 2, the area of the top of the piston 2 exposed to gasdownstream of the constriction point 30 may be increased in order tooffset somewhat the changes in force that the spring 21 exerts on thepiston when changes in the exhaust pressure cause changes in the pistonposition. The ratio of the areas may also be varied in order tocompensate for the additional downward force applied by the velocity ofthe air flowing down from the inlet 4.

The stepper motor can also adjust the position of the spring base 22 tocompensate for the varying spring force.

The effect of the varying spring force may be further reduced by using aspring with a very small spring constant, k, so that changes in theposition of the piston, x, result in only small changes in the forceexerted by the spring. In the preferred embodiment of the invention, aspring made by Century Spring Company of Los Angeles, Calif., part no.H91, which has a k of 0.06 lbs./in. and is 7/8 inches long. Thesesprings work effectively under compression and tension, and are able toapply only a small force in order to offset the low weight of the piston2. In the preferred embodiment, shown in FIG. 7, the length of travel ofthe piston from fully open to fully closed is one-hundredth of an inch.Instead of the spring made by Century Spring Co. discussed above, othersprings may be used, but in order to offset the low weight of thepiston, a very small spring constant is preferred. The spring constantshould be large enough, however, so that it can support the weight ofthe piston without being fully compressed when there is no flow throughthe regulator. This is desirable so that, when a very small amount offlow through the regulator begins, the weight of the piston can beoffset by the spring force. Once the flow begins, the spring base may bepushed up in order to offset the weight of the piston.

FIG. 8 shows a preferred configuration of the spring 21, having its ends31 bent so that the ends are oriented axially. FIGS. 9A, 9B and 9C showa preferred connector for attaching the spring to the bottom of thepiston 2 and may also be used for attaching the spring to the top of thespring base 22. The use of the spring shown in FIG. 8 and the connectorof FIGS. 9A, 9B and 9C permits the spring to be placed under eithercompression or tension. The connector has a lip 34, which defines agroove 35 that extends more than 180°, and a hole 32, located near oneend of the groove 35, for receiving the end 31 of the spring 21. Thediameter of the groove matches the inside diameter of the spring 21.FIG. 9C shows the spring 21 attached to the connector. The location ofthe hole 32 places the spring end 31 just outside of the point where thelast coil enters the groove 35, so that the spring end 31 prevents thelast coil of the spring 21 from slipping out of the groove 35. Thisarrangement prevents longitudinal motion of most of the spring's firstcoil and prevents rotation of the spring. It is also preferred that thestepping motor shaft be keyed to prevent rotation. The advantage of thisattachment is that it provides a secure attachment without adding muchweight to the piston and without using a chemical adhesive.

Since the system shown in FIG. 3 is meant to create only a smallpressure drop from the environment to the plenum 13, it is not usuallypractical to adopt the FIG. 3 system to maintain a constant flow rate byputting a throttling valve between the equipment to create a largepressure drop between the equipment and the plenum 13, and connectingthe reference port 9 to a point upstream of the throttling valve. (Suchan arrangement can nevertheless be used, especially if a heavy piston isused and the pressure differential is large.)

FIGS. 4 and 5 show regulators that have hingedly mounted pistons 2.These regulators, like the regulators shown in FIGS. 1-3, can be used toregulate the partial vacuum in a piece of process equipment, a fume hoodor other region, by attaching their inlets 4 to the equipment, theiroutlets 8 to a vacuum source, and their reference ports 9 to the processroom.

The FIG. 4 regulator's piston 2 has attached to it a slidable weight 90,which can be moved along the weight guide-rod 91 by a stepper motor,like the weight shown in FIG. 9 of U.S. Pat. No. 5,000,221. Moving theweight 90 to the right causes the piston 2 to seem heavier and causesthe constriction point 30 to widen (assuming other parameters--referencepressure, vacuum-source strength and the equipment's flow impedance--donot change). Moving the weight 90 to the left causes the piston 2 toseem lighter and causes the constriction point 30 to narrow. By movingthe weight 90 far enough to the left of the hinge 84, the apparentweight of the piston can be made very small. The slidable weight 90provides a torque about pivot point 84 that opposes the torque createdby the weight of the piston 2. By making the weight of the piston 2 seemsmall, a small pressure differential between the plenum 13 and thereference chamber 17 can be maintained, which in turn can be used tomaintain a small partial vacuum in the equipment.

For the regulator to operate properly in the orientation shown in FIG.4, the weight 90 should not be moved so far to the left of the hinge 84that the piston 2 will rotate up (counterclockwise) when there is noflow through the regulator. In an alternative embodiment, the whole FIG.4 regulator may be turned 90° counterclockwise. In such an orientationthe weight of the piston 2 does not tend to widen the constriction point30. The weight 90 should be far enough below the hinge 84 that thepiston 2 rests against the stop 33 when there is no flow through theregulator. In such an orientation, it is the weight 90 that provides theforce to widen the constriction point 30.

The FIG. 5 regulator has a compressed spring 21 that performs like thespring 21 in the FIG. 2 regulator. The spring base 22 can be adjusted upand down as in the FIG. 2 regulator. The spring 21 exerts a force on thebottom of the piston 2 that counteracts the weight of the piston 2. Thepiston's weight is the restoring force, which tends to widen theconstriction point 30. In other words, the restoring torque about thepivot point 84 created by the weight of the piston 2 is opposed by thetorque created by compressed spring 21. The spring 21 performs the samefunction (reducing the apparent weight of the piston) as the slidableweight in the FIG. 4 regulator when the slidable weight is to the leftof the pivot point 84.

In addition to springs and weights, other means for applying a torqueinclude the use of a DC electric motor in a stalled condition (which isessentially the use of electromagnetic force) and the use of a pistonand cylinder arrangement as may be found in a pneumatic or hydrauliccontrol system. The various means for creating torques can be used toincrease or offset the restoring torque.

As in the regulators shown in FIGS. 1 and 2, the frontal and distalfaces, 15 and 16, of the regulators shown in FIGS. 4 and 5 are exposedto fluid pressures in the plenum 13 and reference-pressure chamber 17,respectively. When the regulator is operating properly, the pressure inthe reference-pressure chamber 17 is greater than the pressure of airflowing through the plenum 7. Therefore, the difference between thepressure forces on the piston's frontal and distal faces results in anadditional torque in a counterclockwise direction around the pivot point84. When the FIG. 5 system is at equilibrium with fluid flowing throughthe regulator, the torques caused by the spring 21, theplenum/reference-chamber pressure differential, and the weight of thepiston 2, balance out, so that the piston floats and provides thenecessary impedance to the flow at constriction point 80 to maintain thedesired partial vacuum in the equipment.

Although the regulators shown in FIGS. 4 and 5 each have a singleconstriction point 30, the split-airfoil impeder shown in applicationsSer. Nos. 07/850,767 and 07/965,907, or the impeder made of grates shownin application Ser. No. 07/965,907, may be used, thereby providingseveral constriction points in each regulator.

FIG. 6 shows a variation of the FIG. 1 regulator. Like the FIG. 1regulator, fume-laden air can flow through the inlet 4 from a region,such as process equipment or a fume hood, which is to be kept at aconstant partial vacuum with respect to an environment, which isconnected to the reference chamber 17 by means of a reference port 9.The outlet 8 of the regulator is attached to a vacuum source. The flowof the fume-laden air is represented by the wide, white arrows 48.

In this variation, a buffering gas which is harmless to humans, such asnitrogen, is introduced under pressure to the regulator through port 66.The buffering gas passes through the piston-mounting structure 65 to agroove 67 around the perimeter of the piston-mounting structure 65. Thebuffering gas then moves through the space between the piston-mountingstructure 65 and the skirt 29 of the piston 2 in two directions. Thepath of the buffering gas is shown by the narrow, black arrows 49. Someof the buffering gas passes into the reference-pressure chamber 17, andsome of the buffering gas passes into the flow of the fume-laden air andthen through the regulator's output 8. This flow of buffering gas intothe flow of the fume-laden air prevents any of the fumes from passingthrough the reference chamber 17 and the reference port 9. According tothe safety standards used by several companies, a gas velocity of 300ft./sec. prevents the backflow of fumes against the flow. Thus, if thevelocity of the nitrogen, or other buffering gas, is more than 300ft./sec., the risk of fumes passing through the reference port into theenvironment is minimal.

By making the passage between the piston skirt 29 and thepiston-mounting structure 65 narrow, and the reference-pressure conduit9 wide and short, one can minimize the difference in pressure betweenthe reference chamber 17 and the environment. It is important tominimize this pressure difference in order to maintain more effectivelya constant partial vacuum in the region with respect to the environment.It is also desirable to place the groove 67 somewhat near the middle ofthe piston-mounting structure 65, so that the upward drag force on theskirt 29 caused by the buffering gas flowing between the groove 67 andthe reference chamber 17 is counteracted by the downward drag forcecaused by the buffering gas flowing between the groove 67 and theexhaust 8. It is preferred that the groove 67 be slightly above themidpoint of the piston-mounting structure 65, as shown in FIG. 7, sothat the downward drag force of the buffering fluid on the inside of thepiston skirt 29 balances the upward drag force and the momentum of theupwardly flowing buffering fluid striking the bottom face 16 of thepiston 2.

This use of a buffering gas in this way has the additional advantage ofkeeping the area between the piston skirt 29 and the piston-mountingstructure 65 free from particles which can interfere with the movementof the piston. This structure also provides a pressurized-gas (nitrogen)bearing, which permits the regulator to be tilted from its verticalorientation. The non-pressurized air bearings between thepiston-mounting structures 65 and the skirt of the piston 2 of the FIG.1 device do not permit such tilting of the regulator, since the weightof the piston 2 would push the piston skirt against the side ofpiston-mounting structure 65, thereby creating too much friction for thepiston to move easily in response to fluctuations in the pressures ofthe environment and the region. Linear ball bearings may be used as analternative to the nitrogen bearing, in order to permit tilting of theregulator. Of course, the linear ball bearings are not, by themselves,effective keeping fumes from leaking through the reference port orparticles out of the area between the piston skirt 29 and thepiston-mounting structure 65. A flexible diaphragm may be used to sealthe reference chamber 17 from the fume-laden air flowing through theregulator.

By tilting the regulator, the weight of the piston 2 provides a smallerrestoring force. By reducing the restoring force, the pressuredifferential between the plenum 13 and the reference chamber 17, andbetween the region and the environment, is also reduced. Thus, having adirection of movement for the piston which is not vertical can be usedto provide a smaller partial vacuum. The regulator should not be tiltedas much as 90°, or otherwise the weight of the piston 2 will not provideany restoring force, and if it is tilted more than 90° the weight of thepiston will tend to close the constriction point 30.

FIG. 7 shows a regulator that uses a buffering gas, as well as a spring21 (which is represented schematically) to reduce the apparent weight ofpiston 2. In this regulator, the piston skirt 29 is significantly longerthan the diameter of the piston 2. Since the piston 2 is top-heavy (thepiston skirt is thin in order to keep the weight of the piston low),providing a long skirt stabilizes the piston. In addition, forcing thebuffering gas to flow a longer distance from the groove 67 to theexhaust 8 decreases the likelihood of the fume-laden air from backing upthrough the reference-pressure port 9. In addition, the pressure in thereference chamber 17 can be kept very close to pressure of theenvironment (which is connected to the reference port 9) by keeping (1)the distance the buffering gas has to flow from the groove 67 to thereference chamber 17 long, (2) the passage between the groove and thepiston skirt narrow, (3) the distance between the reference chamber 17and the environment short and (4) the passage between the referencechamber and the environment wide.

The regulator shown in FIG. 7 has a dashpot 25 in order to prevent thepiston from oscillating at a harmonic frequency, which a low-weightpiston is prone to do. FIG. 10 shows the dashpot 25 in greater detail.The dashpot 25 is made of two rigid portions which can move in relationto each other: the first portion 26 is attached to the piston 2, and thesecond portion is attached to the piston-mounting structure 65. Eachportion is made of a cylindrical wall 26c, 27c and a radial wall 26r,27r extending perpendicular from the cylindrical wall; for one portionthe radial wall 26r is located inside the cylindrical wall 26c, and forthe other portion the radial wall 27r is located outside of thecylindrical wall 27c. Both radial walls have circular apertures definedtherein. The two dashpot portions 26, 27 together define a chamber 28 ofvarying volume and are precisely machined so as to fit snugly withrespect to each other, but not so tightly as to prevent air fromentering or exiting the chamber 28 as the two portions are moved withrespect to each other. In the dashpot 25 shown in FIG. 10, the innerdiameter of the radial wall 26r of the first portion 26 is only slightlygreater than the outer diameter of the cylindrical wall 27c of thesecond portion 27, and the outer diameter of the radial wall 27r of thesecond portion 27 is only slightly less than the inner diameter of thecylindrical wall 26c of the first portion 26. Preferably, the dashpotportions 26, 27 are made of titanium, which provides low weight,dimensional stability and corrosion resistance.

When the piston 2 moves (because of some change in the systemparameters, such as a change in the strength of the evacuation source)the volume of the dashpot chamber 28 also changes. Because the spacesbetween the two dashpot portions 26, 27 are so small, air cannot rushinto the dashpot chamber 28 too quickly. This arrangement slows anddampens the movement of the piston 2.

FIG. 11 shows how the regulator of FIG. 7 may be used in a system forcontrolling flow through a process region, which may be located inside apiece of process equipment. Air flows from the clean room to the processregion, where noxious fumes, such as hydrochloric acid fumes, may bepicked up by the air. The process region is kept at a pressure slightlybelow that of the clean room (the process room) so as to prevent thefume-laden air from flowing back into the clean room. Creating a largepressure differential between the clean room and the region provides thepotential for air to flow into the region from the clean room at highvelocity. Such high velocity streams can sometimes adversely effect theprocess taking place inside the region.

In some embodiments, it may be desirable to have the pressure in theprocess region slightly greater than the pressure in the clean room, inorder to keep dust from flowing into the process region from the cleanroom. The region must still be connected to an evacuation source inorder to pull the fumes from the process region into the exhaust system,and the region's pressure usually cannot be too much greater than theclean room's pressure, otherwise noxious fumes may flow out of theregion into the clean room. In order to accomplish this, the spring'sbase is moved up so far that the force of the spring on the piston isgreater than the weight of the piston.

As part of the flow-control system, the pressure of the process regionis monitored by a pressure (vacuum) transducer 54, which is attached tothe process region by a tube 53. The measurements taken by thetransducer 54 may be used to determine the proper position of the springbase (item 22 in FIG. 2) so that the desired pressure can be created inthe process region. In order to prevent potentially damaging gases(which may be extremely hot or which may contain corrosive fumes) frombacking up the tube 53 and damaging the transducer 54, and to preventwater vapor to from entering the tube, condensing and effectivelysealing off the transducer from the process region, a buffering gas,such as nitrogen, is provided from valve 61d to the tube 53 underpressure, so as to flow from the transducer 54 to the process region.

Of course, this use of a buffering gas in transducer tube 53 creates ahigher pressure at transducer 54 than in the process region. Thisdifference in pressure may be minimized by keeping the velocity of thebuffering gas low. (In one embodiment, the tube 53 has a cross-sectionalarea of 0.08 cm² and a flow rate of about 0.1 liter/min.) In addition,the readings of the transducer 54 can be adjusted to take into accountthe higher pressure created by the buffering gas flowing through thetube 53. In any case, although the transducer 53 may not be able toaccurately measure the actual pressure in the process region, thetransducer 53 can still measure changes in the pressure of the processregion.

This buffering gas serves a similar purpose to the buffering gasdiscussed above in relation to FIG. 6, which shows the buffering gasbeing introduced to the regulator through port 66. The buffering gasintroduced to the regulator through port 66 buffers the clean room (andthe spring and the dashpot) from noxious fumes, and the buffering gasfed to the transducer tube 53 buffers the transducer 54 from thepotentially corrosive fumes.

The two buffering gases may be provided from the same source, as shownin FIG. 11. A pressurized supply of nitrogen gas (N₂) is provided to aconventional regulator 60b which reduces the pressure to about 5 psi.This regulator 60b provides the buffering gas to three variable valves,one of which 61b provides the gas to port 66, another of which 61dprovides the gas to the transducer tube 53, and the last of which 61cprovides gas for cooling the stepper motor that moves the spring base(item 22 in FIG. 2). By using a regulator 60b and keeping the variablevalves 61b, 61c, 61d set at given settings, fairly constant flow ratescan be maintained through the valves 61b, 61c, 61d. In the preferredembodiment depicted, the gas that passes through valve 61c (theregulator buffering gas and the stepper-motor cooling gas) eventuallyends up in the clean room. Whereas it is particularly important (in thepreferred embodiment) that the regulator's reference port 8 be vented tothe clean room in order to establish a constant pressure differentialbetween the process region and the clean room, thereby causing theregulator buffering gas to flow into the clean room, it is not importantthat the cooling gas pass eventually into the clean room, and thecooling gas may be allowed to flow into another area.

Some of the pressurized nitrogen may also be used as a biasing gas. Ascan be seen in FIG. 11, some of the nitrogen is provided to a secondconventional regulator 60a and then to another variable valve 61a,preferably a needle valve. This biasing gas is then supplied through anannular cavity 50 that distributes the gas evenly around the regulator'sinput 4. The cavity 50 also acts as a condensation trap, removingmoisture from any gas that may be backing up the nitrogen supply line(which can happen if the nitrogen supply is cut off). The cavity's input52 extends into the cavity 50 so as to prevent liquid from flowing backto the needle valve 61a and the regulator 60a if the biasing gas flowwere shut off. The tube connecting the needle valve 61a and the cavity50 is preferably of small diameter to provide resistance to theregulator 60a and to prevent backstreaming of fume-laden gas from theprocess region to the needle valve 61a and the regulator 60a.

Without the biasing gas flow, the flow rate of gas from the process mayvary, for example, from 30 liters/min. to 0.3 l/min.--a variation of100:1. By introducing, say, 15 l/min. of biasing gas into the input ofthe regulator, the variation in flow rate that the regulator must handleis only 3:1 (45 l/min. to 15.3 l/min.).

The biasing gas is particularly useful for a preferred embodiment of theregulator, in which the piston 2 and the inside wall of the housing 3are made of a material, RYTON (made by DuPont), which is hard,dimensionally stable and corrosion resistant, but which does not make agood seal. (In the preferred embodiment of the system the RYTON isplaced on the inside of a canister made of PVDF. VYTON is used for theseals in the system.) By using a biasing gas, the piston 2 and thehousing 3 do not have to make contact in order to properly control flow.The biasing gas also helps prevent backstreaming into the process regionof exhaust materials from other equipment sharing the house-exhaustsystem.

If there is a possibility of a sudden failure of the house-exhaustsystem, it is desirable to be able to quickly turn off the biasing gassupply. Otherwise, if there is no exhaust pulling the biasing gas andthe fume-laden gas through the regulator, the biasing gas may force thefume-laden gas into the clean room.

Another condensation trap (not shown in FIG. 11) may be located upstreamof the regulator's input 4, in order to remove some of the chemicalsused in the process before the gases pass through the regulator and intothe house exhaust system. FIG. 12A shows a preferred version of such acondensation trap. FIG. 12B shows a top view of the thermoelectriccooler used in the condensation trap. Vapor-saturated gas enters theinput of the condensation trap at point A. From point B the gas makescontact with the cold, inner side of a thermoelectric cooler made ofpeltier elements (solid-state cooling devices, which function likethermocouples in reverse, and which are available from Melcor ofTrenton, N.J.) at points D. The condensate is collected on the cold sideof the elements.

From point E the condensate can flow into a U-shaped liquid trap,extending to point F and then point G. Preferably the height from pointF to point E is sufficient to keep the liquid trap filled with liquid.

What is claimed is:
 1. A device, connected to an evacuation means, forregulating in a region a partial vacuum with respect to a referencepressure, the device comprising:a path, through which fluid passes fromthe region to the evacuation means, the path being defined by a rigidwall; a movably mounted piston having a frontal face, which is exposedto fluid in the path and which has a central portion and a rim, and adistal face, which is exposed to a reference pressure, the piston beingdisposed in the path so that the piston may constrict the path at aconstriction point between the rim of the piston and the path's wall,wherein fluid in the path on the evacuation means side of theconstriction point exerts a pressure on the piston in a directiontransverse to the piston's direction of movement, and the piston beingmounted so that the weight of the piston exerts a force on the piston ina direction that tends to widen the path at the constriction point,wherein the piston is disposed in the path so that the fluid flowsradially outward from the frontal face's central portion towards thepiston's rim before flowing through the constriction point; a base thatcan be moved with respect to the portion of the path at the constrictionpoint; and a spring, one end of which is attached to the piston and theother end of which is attached to the base.
 2. A device according toclaim 1, wherein the partial vacuum in the region is with respect to anenvironment's pressure, fluid flows from the environment into the regionand then into the device, and the reference pressure is theenvironment's pressure.
 3. A device according to claim 1, wherein thespring is mounted to the distal face of the piston.
 4. A deviceaccording to claim 1, further includingbiasing-fluid means for providinga biasing fluid to the path upstream of the piston, so as to ensure thatthere is always a substantial flow rate through the device.
 5. A device,connected to an evacuation means, for regulating in a region a partialvacuum with respect to a reference pressure, the device comprising:apath, through which fluid passes from the region to the evacuationmeans; a movably mounted piston having a frontal face, which is exposedto fluid in the path, and a distal face, which is exposed to a referencepressure, the piston being disposed in the path so that the piston mayconstrict the path at a constriction point, wherein fluid in the path onthe evacuation means side of the constriction point exerts a pressure onthe piston in a direction transverse to the piston's direction ofmovement, and the piston being mounted so that the weight of the pistonexerts a force on the piston in a direction that tends to widen the pathat the constriction point; a base that can be moved with respect to theportion of the path at the constriction point; and a spring, one end ofwhich is attached to the piston and the other end of which is attachedto the base, wherein the spring is mounted to the distal face of thepiston; wherein the partial vacuum in the region is with respect to anenvironment's pressure, fluid flows from the environment into the regionand then into the device, and the reference pressure is theenvironment's pressure.
 6. A device, connected to an evacuation means,for regulating in a region a partial vacuum with respect to a referencepressure, the device comprising:a path, through which fluid passes fromthe region to the evacuation means; a movably mounted piston having afrontal face, which is exposed to fluid in the path, and a distal face,which is exposed to a reference pressure, the piston being disposed inthe path so that the piston may constrict the path at a constrictionpoint, wherein fluid in the path on the evacuation means side of theconstriction point exerts a pressure on the piston in a directiontransverse to the piston's direction of movement, and the piston beingmounted so that the weight of the piston exerts a force on the piston ina direction that tends to widen the path at the constriction point;wherein the frontal face has an effective area against which fluid inthe path exerts a pressure, the distal face has an effective areaagainst which the pressure of fluid upstream of the frontal face isexerted, and the effective areas of the frontal and distal faces aresubstantially equal to each other; a base that can be moved with respectto the portion of the path at the constriction point; and a spring, oneend of which is attached to the piston and the other end of which isattached to the base.
 7. A device, connected to an evacuation means, forregulating in a region a partial vacuum, the device comprising:an inletconnected to the region; an outlet connected to the evacuation means; apath bounded by a rigid wall, through which fluid passes from the inletto the outlet, the path not being vented at any point between the inletand the outlet; a piston-mounting structure; a piston, movably mountedon the piston-mounting structure, the piston having a rim, a frontalface, which is exposed to fluid in the path between the inlet and theoutlet, and which has a central portion, and a distal face, which isexposed to a reference chamber having a reference pressure greater thanthe evacuation means's pressure, the piston being disposed in the pathso that the piston may constrict the path between the piston's rim andthe path's wall at a constriction point, wherein the piston is disposedin the path so that the fluid flows radially outward from the frontalface's central portion towards the piston's rim before flowing throughthe constriction point, and wherein the piston has a skirt whichsurrounds a portion of the piston mounting structure; and a spring forexerting a force on the piston, the spring being located within thepiston's skirt and the piston-mounting structure.
 8. A device accordingto claim 7, further including a dashpot having a first portion rigidlyattached to the piston and a second portion rigidly attached to thepiston-mounting structure, the two portions of the dashpot defining achamber of varying volume and being so dimensioned as to permit onlyrestricted fluid communication to the dashpot chamber.
 9. A deviceaccording to claim 8, wherein the base does not rotate as it is beingmoved.
 10. A device, connected to an evacuation means, for regulating ina region a partial vacuum, the device comprising:a path bounded by arigid wall, through which fluid passes from the region to the evacuationmeans; a piston-mounting structure; a piston, movably mounted on thepiston-mounting structure, the piston having a rim, a frontal face,which is exposed to fluid in the path, and a distal face, which isexposed to a reference chamber having a reference pressure, the pistonbeing disposed in the path so that the piston may constrict the pathbetween the piston's rim and the path's wall at a constriction point;and a spring for exerting a force on the piston, the spring beinglocated within the piston and the piston-mounting structure; wherein thepartial vacuum in the region is with respect to an environment'spressure, fluid flows from the environment into the region and then intothe device, and the reference pressure is the environment's pressure,and wherein the spring is under compression so as to exert a force onthe piston in a direction that tends to narrow the path at theconstriction point.
 11. A device according to claim 10, wherein thespring is mounted on a base, which can be moved with respect to theportion of the path at the constriction point, in order to adjust thepartial vacuum.
 12. A device according to claim 10, wherein the frontalface has an effective area against which fluid in the path exerts apressure, the distal face has an effective area against which thereference pressure is exerted, and the effective areas of the frontaland distal faces are substantially equal to each other.
 13. A deviceaccording to claim 10, further includinga dashpot having a first portionrigidly attached to the piston and a second portion rigidly attached tothe piston-mounting structure, the two portions of the dashpot defininga chamber of varying volume and being so dimensioned as to permit onlyrestricted fluid communication to the dashpot chamber.
 14. A deviceaccording to claim 10, further including biasing-fluid means forproviding a biasing fluid to the path upstream of the piston, so as toensure that there is always a substantial flow rate through the device.15. A device, connected to an evacuation means, for regulating in aregion a partial vacuum, the device comprising:a path bounded by a rigidwall, through which fluid passes from the region to the evacuationmeans; a piston-mounting structure; a piston, movably mounted on thepiston-mounting structure, the piston having a rim, a frontal face,which is exposed to fluid in the path, and a distal face, which isexposed to a reference chamber having a reference pressure, the pistonbeing disposed in the path so that the piston may constrict the pathbetween the piston's rim and the path's wall at a constriction point;and a spring for exerting a force on the piston, the spring beinglocated within the piston and the piston-mounting structure; wherein thefrontal face has an effective area against which fluid in the pathexerts a pressure, the distal face has an effective area against whichthe reference pressure is exerted, and the effective areas of thefrontal and distal faces are substantially equal to each other; andwherein the partial vacuum in the region is with respect to anenvironment's pressure, fluid flows from the environment into the regionand then into the device, and the reference pressure is theenvironment's pressure.
 16. A device according to claim 15, furtherincluding biasing-fluid means for providing a biasing fluid to the pathupstream of the piston, so as to ensure that there is always asubstantial flow rate through the device.
 17. A device, connected to anevacuation means, for regulating in a region a partial vacuum, thedevice comprising:a path bounded by a rigid wall, through which fluidpasses from the region to the evacuation means; a piston-mountingstructure; a piston, movably mounted on the piston-mounting structure,the piston having a rim, a frontal face, which is exposed to fluid inthe path, and a distal face, which is exposed to a reference chamberhaving a reference pressure, the piston being disposed in the path sothat the piston may constrict the path between the piston's rim and thepath's wall at a constriction point; a spring for exerting a force onthe piston, the spring being located within the piston and thepiston-mounting structure; and buffering-fluid means for supplying abuffering fluid to a point between the piston and the piston-mountingstructure such that a first portion of the buffering fluid flows intothe path and a second portion of the buffering fluid flows into thereference chamber.
 18. A device according to claim 17, wherein thepartial vacuum in the region is with respect to an environment'spressure, fluid flows from the environment into the region and then intothe device, and the second portion of the buffering fluid flows to theenvironment after flowing through the reference chamber.
 19. A deviceaccording to claim 17, further including biasing-fluid means forproviding a biasing fluid to the path upstream of the piston, so as toensure that there is always a substantial flow rate through the device.20. A device according to claim 18, further including biasing-fluidmeans for providing a biasing fluid to the path upstream of the piston,so as to ensure that there is always a substantial flow rate through thedevice.
 21. A device according to claim 17, further including a dashpothaving a first portion rigidly attached to the piston and a secondportion rigidly attached to the piston-mounting structure, the twoportions of the dashpot defining a chamber of varying volume and beingso dimensioned as to permit only restricted fluid communication to thedashpot chamber.
 22. A device, connected to an evacuation means, forregulating in a region a partial vacuum, the device comprising:an inletconnected to the region; an outlet connected to the evacuation means; apath bounded by a wall, through which fluid passes from the inlet to theoutlet, the path not being vented at any point between the inlet and theoutlet; a movably mounted piston, the piston having a rim and a frontalface, which is exposed to fluid in the path between the inlet and theoutlet, the frontal face having a central area, the piston also having adistal face, which is exposed to a reference pressure greater than theevacuation means's pressure, the piston being disposed in the path sothat the piston may constrict the path between the piston's rim and thepath's wall at a constriction point, and so that fluid flows radiallyoutward from the frontal face's central portion towards the piston's rimbefore flowing through the constriction point, wherein the piston has askirt extending from the rim; and a spring for exerting a force on thepiston, the spring being located within the piston's skirt.
 23. A deviceaccording to claim 22, wherein the partial vacuum in the region is withrespect to an environment's pressure, fluid flows from the environmentinto the region and then into the device, and the reference pressure isthe environment's pressure.
 24. A device according to claim 22, whereinthe spring is mounted on a base, which can be moved with respect to theportion of the path at the constriction point, in order to adjust thepartial vacuum.
 25. A device, connected to an evacuation means, forregulating in a region a partial vacuum, the device comprising:a pathbounded by a wall, through which fluid passes from the region to theevacuation means; a movably mounted piston, the piston having a rim anda frontal face, which is exposed to fluid in the path, the frontal facehaving a central area, the piston also having a distal face, which isexposed to a reference pressure, the piston being disposed in the pathso that the piston may constrict the path between the piston's rim andthe path's wall at a constriction point, and so that fluid flowsradially outward from the frontal face's central portion towards thepiston's rim before flowing through the constriction point; and a springfor exerting a force on the piston; wherein the distal face of thepiston is exposed to a reference chamber and the piston is mounted on apiston-mounting structure, the device further including buffering-fluidmeans for supplying a buffering fluid to a point between the piston andthe piston-mounting structure, such that a first portion of thebuffering fluid flows into the path and a second portion of thebuffering fluid flows into the reference chamber.
 26. A device accordingto claim 25, further including biasing-fluid means for providing abiasing fluid to the path upstream of the piston, so as to ensure thatthere is always a substantial flow rate through the device.
 27. A devicefor controlling the flow of a first fluid, the device comprising:apassageway through which the fluid flows; a piston movably mounted sothat as the piston moves the piston applies a varying resistance to theflow through the passageway; a piston-mounting structure on which thepiston is mounted, the piston-mounting structure and the piston defininga chamber in fluid communication with an environment; and second-fluidmeans for supplying a second fluid to a point between the piston and thepiston-mounting structure, such that a first portion of the second fluidflows into the passageway and a second portion of the second fluid flowsinto the chamber; wherein the piston includes a skirt which surrounds aportion of the piston-mounting structure, and the point to which thesecond-fluid supplying means supplies the second fluid is between thepiston-mounting structure and the piston's skirt.
 28. A device accordingto claim 27, wherein the piston restricts the flow through thepassageway at a constriction point between the piston and a wall of thepassageway, such that the first portion of the second fluid flows intothe passageway downstream of the constriction point.
 29. A deviceaccording to claim 28, wherein fluid flows from an environment, throughthe passageway and then to a vacuum source, and the reference pressureis the environment's pressure.
 30. A device according to claim 29,wherein the piston-mounting structure is tilted from a verticalorientation.
 31. A device according to claim 27, further includingbiasing-fluid means for providing a biasing fluid to the path upstreamof the piston, so as to ensure that there is always a substantial flowrate through the device.
 32. A device, for controlling the flow of afirst fluid, the device comprising:a passageway through which the fluidflows; a piston movably mounted so that as the piston moves the pistonapplies a varying resistance to the flow through the passageway; apiston-mounting structure on which the piston is mounted, thepiston-mounting structure and the piston defining a chamber in fluidcommunication with an environment; and second-fluid means for supplyinga second fluid to a point between the piston and the piston-mountingstructure, such that a first portion of the second fluid flows into thepassageway and a second portion of the second fluid flows into thechamber; wherein the piston-mounting structure is tilted from a verticalorientation.
 33. A device for regulating the flow of fluid from anenvironment, where the fluid is substantially still, through a region,then through the device, and to a vacuum source, so as to maintain asubstantially constant partial vacuum in the region with respect to theenvironment, the device comprising:a passageway through which the fluidflows from an inlet in fluid communication with the region to an outletin fluid communication with the vacuum source; a piston movably mountedadjacent the passageway, so that as the piston moves the piston appliesa varying resistance on the flow through the passageway at aconstriction point, the piston having a frontal face exposed to fluidflowing through the passageway upstream of the constriction point, and adistal face opposite the frontal face; and a piston-mounting structureon which the piston is mounted, the piston-mounting structure and thepiston's distal face defining a chamber in fluid communication with anenvironment, the piston-mounting structure being tilted from a verticalorientation, so that the piston moves along a line offset from vertical,and so that the weight of the piston tends to lower the resistance tothe flow, the piston-mounting structure including bearing means forreducing friction between the piston and the piston-mounting structure.34. A device according to claim 33, wherein the bearing means includesbuffering-fluid means for supplying a buffering fluid to a point betweenthe piston and the piston-mounting structure, such that a first portionof the buffering fluid flows into the passageway downstream of theconstriction point and then to the vacuum source and a second portion ofthe buffering fluid flows into the chamber and then the environment. 35.A device according to claim 34, wherein the piston includes a skirtwhich surrounds a portion of the piston-mounting structure, and thepoint to which the buffering-fluid means supplies the buffering fluid isbetween the piston-mounting structure and the piston's skirt.
 36. Adevice, connected to an evacuation means, for regulating in a region apartial vacuum with respect to a reference pressure, the devicecomprising:a path, through which fluid passes from the region to theevacuation means; a movably mounted piston having a frontal face, whichis exposed to fluid in the path, and a distal face, which is exposed toa reference pressure, the piston being disposed in the path so that thepiston may constrict the path at a constriction point, wherein fluid inthe path on the evacuation means side of the constriction point exerts apressure on the piston in a direction transverse to the piston'sdirection of movement, and the piston being mounted so that the weightof the piston exerts a force on the piston in a direction that tends towiden the path at the constriction point; a base that can be moved withrespect to the portion of the path at the constriction point; a steppermotor for moving the base; and a spring, one end of which is attached tothe piston and the other end of which is attached to the base.